zephyr: migrate slot 0 validation to Kconfig

Handle CONFIG_BOOT_VALIDATE_SLOT0 in mcuboot_config.h, and switch to
setting it with an OVERLAY_CONFIG fragment in the test Makefile.

Signed-off-by: Marti Bolivar <marti@opensourcefoundries.com>
diff --git a/samples/zephyr/Makefile b/samples/zephyr/Makefile
index 464cf6e..ec14c46 100644
--- a/samples/zephyr/Makefile
+++ b/samples/zephyr/Makefile
@@ -257,7 +257,7 @@
 # reset: hello1 runs
 test-no-bootcheck: clean
 	$(MAKE) \
-		BOOTLOADER_CONFIG="-DCONF_VALIDATE_SLOT0=NO" \
+		BOOTLOADER_OVERLAY_CONFIG=$(PWD)/overlay-skip-slot0-validate.conf \
 		SIGNING_KEY=../../root-ec-p256.pem \
 		all